The present invention relates to a method of making a high speed steel drill provided with a PVD-coating. By subjecting the drill to an electropolishing operation prior to depositing the coating, an unexpected decrease in tool wear has been obtained.
A substantial number of drilling operations in industry are today performed using drills of high speed steel, (HSS). For operations demanding more wear resistance and for drilling with high cutting speed, drills of solid cemented carbide or drills equipped with cemented carbide inserts are used. The use of such drills is increasing at the expense of HSS drills. For conventional high speed drills, the wear resistance can be improved to a considerable degree by providing them with a coating of a wear resistant, about 5 .mu.m thick, layer of usually TiN which, in addition, gives the drills a nice golden color. The coating has to be applied by some PVD-process operating at a low temperature, below the softening temperature of the hardened and tempered HSS.
HSS drills are made from cylindrical bars in which the flute and edge are formed by grinding. A ground HSS drill often has a relatively rough surface in the flute, grinding burrs along the cutting edge and possibly thermal damages (grinding burns) in the HSS surface, resulting in a hardened and embrittled surface layer. The cutting edge on the ground drill is further often very sharp which increases the propensity for crack formation, chippings and edge fractures, especially when drilling in hard work materials.
After grinding of flutes and drill point, the coating is generally applied directly without further treatment other than conventional cleaning in order to remove dust, oil and other impurities from the grinding.
Electrolytic smoothing or deburring is a commonly employed technique. Two well-known processes are called electrochemical deburring and electropolishing. U.S. Pat. No. 4,405,422 discloses methods for electrolytic deburring of copper or copper alloys and U.S. Pat. No. 4,411,751 of steel or aluminum alloys. Swedish patent application SE 9404326-2 discloses a method for edge rounding of cutting tool inserts of cemented carbide or titanium based carbonitride alloys. An electrolytic method is used with an electrolyte which provides an even removal of both binder phase and hard constituent phases containing perchloric (HClO.sub.4) or sulphuric (H.sub.2 SO.sub.4) acid, 2-15 vol %, in methanol or other organic liquid.
The object of the present invention is to further improve the performance of coated HSS-drills by an electropolishing treatment.
In one aspect of the invention there is provided a high speed steel tool with carbides in the steel matrix, a PVD coating on at least a portion of the tool and at least one cutting edge wherein the cutting edge has a radius of 15-25 .mu.m and the carbides in the high speed steel are in contact with the coating.
In another aspect of the invention there is provided a method of making a PVD coated high speed steel tool with at least one cutting edge wherein the tool is subjected to an electropolishing treatment such that a cutting edge radius of 15-25 .mu.m is obtained and a light etching such that the carbides in the high speed steel are revealed and the tool is then coated by PVD.